This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Review Cite This: Chem. Rev. 2020, 120, 200−268 pubs.acs.org/CR Photo- and Redox-Driven Artificial Molecular Motors Massimo Baroncini,*,†,‡ Serena Silvi,*,†,§ and Alberto Credi*,†,‡ † CLAN-Center for Light Activated Nanostructures, Istituto ISOF-CNR, via Gobetti 101, 40129 Bologna, Italy ‡ Dipartimento di Scienze e Tecnologie Agro-alimentari, Universitàdi Bologna, viale Fanin 44, 40127 Bologna, Italy § Dipartimento di Chimica “G. Ciamician”, Universitàdi Bologna, via Selmi 2, 40126 Bologna, Italy ABSTRACT: Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature’s choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed. CONTENTS 5. From Movements to Functions 237 5.1. Control of Chemical and Physical Properties 238 1. Introduction 201 fi 5.1.1. Transfer of Chirality 238 1.1. Terms and De nitions 201 5.1.2. Catalytic Properties 240 1.2. Directional Motion in Molecular Systems 202 5.1.3. Biological Functions 241 1.2.1. Energy Ratchet 202 5.2. Transmission of Directed Motion within 1.2.2. Information Ratchet 204 Molecular Systems 243 1.3. Energy Supply and Autonomous Behavior 205 5.3. Locomotion at the Nanoscale 246 1.3.1. Chemical Stimulation 205 5.3.1. Molecular Swimmers 246 1.3.2. Electrical Stimulation 207 5.3.2. Surface-Roving Nanovehicles 247 1.3.3. Optical Stimulation 207 5.4. Controlled Transport of Molecular Cargos 248 2. Redox-Driven Motors 209 5.4.1. Systems Based on Linear Molecular 2.1. General Considerations 209 Switches 248 2.2. Useful Redox Processes 210 5.4.2. Systems Based on Rotary Molecular 2.3. Supramolecular Pumps 210 Switches 249 2.4. Molecular Rotary Motors 213 5.5. Amplification of Motion by Collective Strat- 3. Photochemically Driven Motors 216 egies 250 3.1. General Considerations 216 5.5.1. Doped Liquid Crystals 251 3.2. Useful Photoinduced Processes 216 5.5.2. Self-Assembled Systems 252 3.3. Supramolecular Pumps 221 5.5.3. Systems with Covalently Linked Molec- 3.4. Molecular Walkers 222 ular Motor Units 254 3.5. Catenane Rotary Motors 224 6. Conclusions and Outlook 255 3.6. Molecular Rotary Motors 226 Author Information 257 3.6.1. Overcrowded Alkenes 226 Corresponding Authors 257 3.6.2. Hemithioindigos 232 ORCID 257 3.6.3. Imines 233 Notes 257 3.6.4. Other Systems 235 3.7. Photochemically Triggered Chemically Driv- en Motors 235 Special Issue: Molecular Motors 4. Motors with Combined Photo- and Redox Activation 237 Received: May 9, 2019 Published: August 15, 2019 © 2019 American Chemical Society 200 DOI: 10.1021/acs.chemrev.9b00291 Chem. Rev. 2020, 120, 200−268 Chemical Reviews Review Biographies 257 Nevertheless, the construction and operation of systems with Acknowledgments 257 a level of structural and functional sophistication close to that of References 257 natural molecular motors is still a prohibitive task. What chemists have been doing is making simple prototypes consisting of a few molecular components that can move relative to one another in a controllable way and tackling the 1. INTRODUCTION challenging problems posed by the interfacing of such devices Living organisms are equipped with a myriad of complex with the macroscopic world in terms of energy supply, molecular assemblies, called motor proteins, that work inside information exchange, and in more recent years, the task cells for ordinary needs like machines in a manufacturing performed. The many remarkable achievements in this area have − 24−41 plant.1 4 Scientific progress in the past decades has shown that been evidenced in several reviews (including some focused 42−46 47−54 these extremely small motors are in fact the most useful on redox-driven and light-driven systems), themed 55−58 59−61 62−65 machines for humankind, because they are tasked with executing issues, edited books, and monographs. The crucial processes for life. The most important and best known scientific value of molecular machines was highlighted by the motor proteins are those of the myosin5 and kinesin6 families, award of the 2016 Nobel Prize in Chemistry to Jean-Pierre ATP synthase,7 and the bacterial flagellar motor.8 Myosin and Sauvage,66 Fraser Stoddart,67 and Ben Feringa68 well before kinesin are linear motors powered by a chemical reaction these systems could impact significantly on people’s lives.69 namely, the hydrolysis of adenosine triphosphate (ATP)and Clearly, besides the relevance for fundamental science, the are responsible for, among other functions, contraction and Nobel Committee recognized the huge potential of molecular relaxation of muscles and transport of cargos around the cell, machines for major progress in several areas of technology and respectively. ATP synthase and the bacterial flagellum are rotary medicine, with ground-breaking applications limited only by motors, both driven by transmembrane proton gradients, whose imagination.70 functions are manufacturing of ATP in all organisms and This review will cover the progress achieved in the past 5 years locomotion in some classes of bacteria, respectively. Another in the area of molecular motors operated by redox and/or remarkable example is DNA polymerase, which moves along photochemical processes. Molecular motors are a subcategory of DNA while performing transcriptions.9,10 Several other key molecular machines (see section 1.1 for definitions). In order to biological processes, such as protein folding and unfolding,11 are provide a complete and meaningful representation of this rapidly − driven by molecular motors.1 4 Surely, these biomolecular growing area, studies prior to 2014 that are particularly mechanical devices provide a compelling demonstration of the significant for historical, tutorial, or scientific reasons or are feasibility and utility of nanotechnology and constitute a sound still the sole examples of a given type of device will also be scientific basis for attempting the construction of wholly artificial included. We will focus on systems in which the motor function machines that operate at the molecular scale. is inherent either to the individual molecule or, in the case of Although the latter idea was proposed for the first time by supramolecular complexes, to a minimal number of molecular Richard Feynman in his landmark 1959 talk,12 only in the past components. Therefore, materials and devices whose motorlike three decades has the progress in several areas of chemistry behavior is determined by the collective action of molecular primarily, the tremendous development of new synthetic, switches (which are not motors) and/or rectification features analytical, and computational tools to make and investigate arising from the anisotropy of the material or its environment complex molecular assembliesenabled its practical realization. will not be considered here. Interested readers can refer to recent − − In the 1980s, research in supramolecular chemistry triggered the articles71 82 and reviews83 91 on redox- and photoinduced creativity of the chemical community, and the possibility that the mechanical actuation and propulsion in nanostructured systems concepts of macroscopic device and machine, typical of and materials. engineering, could be transferred to the molecular level started 1.1. Terms and Definitions to arise.13,14 Specifically, it was proposed that molecules could be used as building blocks to construct nanoscale devices and As discussed in the previous section, the macroscopic concepts machines by a bottom-up approach.15 In the following years, as a of device and machine can be transferred to the molecular level. result of the rapid growth of supramolecular chemistry16 A molecular device is defined as an assembly of a discrete particularly in its broader meaning of the chemistry of number of molecular components designed to execute a 15,16 multicomponent speciesand mastering of the mechanical predetermined task. While each part performs a single act, bond between molecules,17,18 it became clear that the molecular the multicomponent (supramolecular) assembly can achieve a bottom-up approach offers virtually unlimited possibilities for more complex function that results from the cooperation of the the realization of nanoscale machines. Single-molecule micros- molecular components and is therefore inaccessible to them copies have provided direct observations of molecules as individually. A machine is a device characterized by the use of nanoscale objects endowed with size and shape that can move, mechanical power, i.e., controlled motion;92 analogously, a interact, and react. Indeed, the visualization by fluorescence molecular machine is a type of molecular device in which the microscopy of the rotation of a single molecular filament driven components can be set in motion relative to one another in a 19 by an F1-ATP synthase motor was not only a remarkable controlled manner, resulting in the potential ability to carry out a research achievement but also an exciting moment for function.36,38,62,63 nanoscientists.